The present invention relates to an implant of the type with an outer cone and having means for releasing an element placed on the outer cone, such as, for example, a joint ball.
In the treatment of human joints which are heavily impaired in function by trauma or illness, an endoprosthesis has found general acceptance. In recent years, endoprostheses have increasingly been provided with a cone onto which various heads can be plugged. These conical plug-in connections provide a very strong connection, even with very small attachment forces. However, in doing this, two primary problems appear:
1. After the ball of the joint is applied to the cone neck and the joint is repositioned, it is frequently noted during surgery that the length of the neck requires adjustment. To make this adjustment, the previously connected ball must be removed.
2. During subsequent surgery, it is often not necessary to replace the prosthesis stem. However, due to damage from frictional forces involved during use, it is advisable to also replace the used ball with an undamaged one.
The ball removing devices presently available on the market can each be used only for some of the implants and, in most cases, do not permit release of the plug-in ball without applying bending moments or forces to the shaft.
An implant of the above-mentioned type is disclosed in German Offenlegungsschrift (laid open patent application) No. 3,319,916, which shows an arrangement in which the joint ball is released by means of a nut which is screwed onto a threaded projection provided below the joint ball by "unscrewing" the nut by means of a tool in the direction toward the joint ball. This arrangement has the drawback that the nut provided directly below the joint ball in some cases limits the mobility of the joint or weakens the shaft perimeter in its outer regions which regions are of particular importance for its strength.